Techniques for generating images as well as various techniques in which image analysis is used have been suggested because of the development of recent image processing techniques and information processing apparatuses. Among them, techniques for extracting the contour of a target object play key roles in a wide range of fields such as visual tracking, computer vision, and analysis and retouching of medical images. The contour of a target object in an image can be considered as a part of an edge. Thus, an edge extracting filter is often used for the extraction of contour lines. For example, in the condensation (conditional density propagation) algorithm for visual tracking techniques, a finite number of particles are used to express the probability distribution of the existence of a target object, and the subsequent existence probability distribution is estimated by matching with an edge image a candidate contour, which is determined by one particle, having the same shape as the target object and then by observing the likelihood of each particle (see, e.g., non-patent documents 1-3).
At the same time, as a technique for extracting contour lines, active contour models (Snakes) have been proposed where the contour model of a target object is expressed by a closed curve, and the contour of the target object is estimated by deforming the closed curve so that an energy function defined in advance is at a minimum (see patent documents 1-2).    [Non-patent document 1] Contour tracking by stochastic propagation of conditional density, Michael Isard and Andrew Blake, Proc. European Conf. on Computer Vision, vol. 1, pp. 343-356, Cambridge UK (1996)    [Non-patent document 2] CONDENSATION—conditional density propagation for visual tracking, Michael Isard and Andrew Blake, Int. J. Computer Vision, 29, 1, 5-28 (1998)    [Non-patent document 3] ICondensation: Unifying low-level and high-level tracking in a stochastic framework, Michael Isard and Andrew Blake, Proc 5th European Conf. Computer Vision, 1998    [Patent document 1] JP 9-138471    [Patent document 2] JP 8-329254